1. Field of the Invention
The present invention relates to a bumper reinforcement member configuring a bumper of a vehicle.
2. Description of the Related Art
A bumper of a vehicle is configured by a bumper reinforcement member for absorbing impact, and a bumper cover for covering the bumper reinforcement member. The bumper reinforcement member absorbs impact by deforming a hollow section (plastic deform of a face that forms the section). In general, a superior bumper has a high maximum value (peak load) in a three-point bending test applying a load at one point and supports a test piece at the other two points, a larger deformed amount and a larger value of integral (in other words, impulse or gross amount of energy absorption). An energy absorption property in the bumper reinforcement member can be enhanced by thickening each face that is plastic-deformed, using a high strength material, or making the cross section complicating.
JP2005-170234A discloses a bumper reinforcement member (impact absorption member) including a plurality of vertical walls having different heights (width in a front and back direction of a vehicle) and a horizontal wall that couples the vertical walls to each other. The bumper reinforcement member is formed with a step at the horizontal wall (4, 5, 9 in FIG. 1 of JP2005-170234A). The vertical walls are arranged in a positional relationship of interfering with each other when buckled or crushed by impact. JP2005-170234A says that, according to such a bumper reinforcement member, the peak load that occurs first (initial peak load) becomes small and the energy absorption amount can be enhanced since the impact is applied in a step wise manner.
The bumper reinforcement member disclosed in JP2005-170234A has a problem in that the deformation of each vertical wall is not uniform as the vertical wall is long. Therefore, the vertical walls may not necessarily interfere when buckled or crushed. Even if the vertical walls are assumed to interfere with each other, the vertical walls are believed to not interfere with each other unless the respective vertical walls are greatly buckled or crushed since each vertical wall is greatly spaced apart. Furthermore, as it is clear from the fact that the bumper reinforcement member has a plurality of peak loads (see graph of FIG. 5 of JP2005-170234A), a stable energy absorption property may not be obtained as a whole. Moreover, since the peak loads appear in plural times, the energy absorption amount inevitably becomes small as a whole.
JP2003-237507A discloses a bumper reinforcement member including a main reinforcement member 1 supported to a vehicle body side, and a supplementary reinforcement member 3 to be attached to a front face of the main reinforcement member. The supplementary reinforcement member has a substantially concaved sectional portion 2 and the main reinforcement member has a concaved groove portion 5. The substantially concaved. sectional portion 2 is in contact with the concaved groove portion 5. The bumper reinforcement member disclosed in JP2003-237507A does not require special members. The structure is also easy to process. Changes in material and special production facilities are unnecessary, and increase in cost can be suppressed. Furthermore, the energy absorption amount is enhanced while suppressing local buckling.
The bumper reinforcement member of JP2003-237507A is designed so that a high peak load appears only once. In this configuration, a stable energy absorption property and a large energy absorption amount are obtained, compared to JP 2005-170234A. However, since the supplementary reinforcement member is attached to the front face of the main reinforcement member, and the supplementary reinforcement member greatly projects out from the front face of the main reinforcement member, the bumper reinforcement member inevitably becomes large. Increase in weight also becomes a problem. Variety of design is also limited. Thus, the bumper reinforcement member disclosed in JP2003-237507A is difficult to use in light automobiles to which weight limits and layout limits are strictly imposed and automobiles having high designability.
JP2004-074834A, discloses a bumper reinforcement member including a main reinforcement member 3 of a rear face opened sectional structure with a front face 9, an upper lateral face 10 and a lower lateral face 11, and an supplementary reinforcement member bridged to the upper and lower lateral faces from the front face of the main reinforcement member. The supplementary reinforcement member has a mountain folded portion 14 on the front face 9. The main reinforcement member 3 includes, on the front face, a front face groove 2 having a concaved cross section with a groove bottom face 12 and a groove lateral face 11, and the supplementary reinforcement member 1 is bridged to the upper and lower lateral faces from the front face groove. The bumper reinforcement member disclosed in JP2004-074834A has a rear face opened sectional structure (structure in which the rear face is opened). The bumper reinforcement member has a high peak load and a large energy absorption amount that are not inferior to the bumper reinforcement member of a closed sectional structure.
The bumper reinforcement member disclosed in JP2004-074834A is not as enlarged as the bumper reinforcement member disclosed in JP2003-237507A since the supplementary reinforcement member is arranged inside the main reinforcement member. Therefore, the bumper reinforcement member of JP2004-074834A does not impose a design limitation. JP2004-074834A gives importance to controlling the deformation of an opened sectional structure. In other words, the energy absorption property of the same extent as the bumper reinforcement member of the closed sectional structure is merely ensured. Therefore, the bumper reinforcement member disclosed in JP2004-074834A does not increase the peak load or increase the energy absorption amount compared to the bumper reinforcement member of the closed sectional structure.
The energy absorption property in the bumper reinforcement member can be enhanced by thickening each face that is plastic-deformed, using a high strength material, or making the cross section complicating. However, the bumper reinforcement member is enlarged and the weight becomes excessively large if each face is thickened. The manufacturing cost becomes high if the high strength material is used. Making the cross section complicating deprives variety of design of the bumper reinforcement member and has a possibility of influencing the design of the entire vehicle. Therefore, miniaturization, reduction in manufacturing cost, and simplification of the cross section become important issues in enhancing the energy absorption property.